This invention relates to a compact fluorescent lamp comprising a circuit board with an operating circuit, and, more particularly, to a construction for holding the circuit board in the lamp.
Low-pressure discharge lamps compatible with incandescent lighting fixtures, the so-called compact fluorescent lamps, include an operating circuitry necessary for their operation. This electronic ballast is placed in the base housing between the threaded or bayonet lamp base shell and the cap holding the discharge tubes. As a consequence, it increases the size of the lamp. This increase in size is disadvantageous from the point of view of the application of the lamp, so it is desirable to make this increase smaller. One way of size-reduction is to increase the operating frequency which results in an operating circuitry smaller in size. However, the increase of frequency is limited by interference elimination standards protecting the environment from electromagnetic radiation. Another way of reducing the size of the circuitry is securing the circuit board supporting the electronic ballast perpendicularly to the longitudinal axis. Such an arrangement is described in U.S. Pat. No. 4,695,767. This patent illustrates that the exhaust tubes extending from the discharge tubes must be taken into account if the circuit board is placed perpendicularly to the longitudinal axis of the lamp. The sealed ends of the exhaust tubes must not knock against either electronic components or the printed circuit board supporting the mounted electronic components. In order to avoid this, a hole should be cut in the printed circuit board at the place where the exhaust tube is extending just as it is shown in the above patent. Such holes, however, make the surface of the printed circuit board where components can be mounted smaller. This fact in turn will require a larger size of the printed circuit board, and will result in a wider lamp. This problem can be resolved if the side wall of the plastic cap supporting the printed circuit board is made longer, i.e. long enough to keep the ends of the exhaust tube at a sufficient distance from the circuitry and its printed circuit board. This solution has the disadvantage that during the heating necessary for the setting of the cement fixing the discharge tube ends, the longer side wall of the cap can get deformed uncontrollably. The danger of deformation can be reduced by using a longer heating session of a lower temperature, but this increases the cycle time of manufacturing.
Thus there is a particular need for a compact fluorescent lamp structure that brings a solution to the above problem. That is, a construction for compact fluorescent lamps containing an electronic ballast is required that permits to safely perform the setting of the cement fixing the discharge tube to the cap even if the electronic ballast is mounted on a printed circuit board which is perpendicular to the longitudinal axis of the lamp.
In an exemplary embodiment of the invention, a compact fluorescent lamp comprises at least one discharge tube convoluted to have discharge tube legs substantially parallel to a longitudinal axis. At least two of the discharge tube legs include lead-in-wires. The lamp also comprises a plastic cap which is a hollow body defined by a cover wall receiving the discharge tube legs through holes formed therein and a side wall. A bonding material is applied between the discharge tube legs and an inside portion of the cover wall of the plastic cap. The bonding material requires a heat treatment for setting. A ballast circuitry is located at least partly on a circuit board. A plastic insert piece is disposed in the plastic cap for holding the circuit board substantially perpendicularly to the longitudinal axis at a distance from the discharge tube legs. The lamp further comprises a base housing which includes a base shell and connects to the side wall of the plastic cap.
One advantage of this construction is that the use of the plastic insert piece permits shortening the side wall of the plastic cap since the insert piece is the component part which holds the circuit board at a distance from the discharge tube legs rather than the side wall of the cap. This distance is sufficient to avoid harmful interference between the tube legs and the circuit board. Owing to the shortened side wall of the cap, the bonding material applied to an inside portion of the cover wall of the cap can be set by a heat treatment where the source of the heat is closer to the bonding material. Consequently, a shorter heating session is required for setting the bonding material and the side wall of the cap and the elements thereof required for connecting to the base housing will not get deformed by the heat uncontrollably.
Another advantage is that the use of the insert piece results in the shortening of the plastic base housing of the lamp.
A further advantage is that the plastic insert piece can be exploited for other purposes which will be explained in the detailed description of the invention.